Interested in going to Mars? You may want to think again. A measuring tool on the Curiosity rover revealed that traveling through deep space for an extended period of time would expose astronauts to massive amounts of radiation.

The Curiosity spacecraft made a great test vehicle, as it is similar to that which would be used by astronauts making the same journey. NASA says the radiation dose astronauts would be exposed to is roughly equivalent to getting a full-body CT scan every five days. But it’s almost unavoidable, as protecting astronauts would require a heavy-duty radiation shield, which is “almost certainly impractical–it would be too heavy and therefore costly,” Geraint Jones, a space scientist at University College London, tells the Guardian.

So what’s the solution? We have to find a way to improve propulsion technology, allowing us to fly to Mars much faster. NASA is busy developing next-gen rockets for exactly this sort of mission. There’s also an independent mission being planned that would avoid some of the radiation issues, but that’s because it’s a one-way trip.